It is known to provide an optical plug connection for optical measuring devices for the transmission of light signals to or the transmission of light signals from the optical measuring device utilizing light waveguides, i.e. optical fibers. A plug connector can be provided on the housing of the measuring device, in particular for joining an internal light waveguide with an external light waveguide, i.e. for providing the optical coupling between them. Conventional measuring devices utilizing optical connectors of this type operate exclusively on the butt-contact principle. The light conducting cores of the two light waveguides to be interconnected lie precisely aligned and as tightly as possible against one another.
Indeed, most plug connections of this type provide means for pressing the end face of one of the light waveguides against the end face of the other light waveguide to be coupled to the first.
If the system is to include an optical transmitter with a large coherence length, for example a laser diode, and is to use glass fibers as the light waveguides, the conventional approach has been to ensure contact between the optical fiber cores to be coupled for two reasons:
a) If an air gap would exist between the two fiber ends, a resonator would be formed because of the double glass-air reflection (glass-air interface/air-glass interface) which exists on both sides of the air gap. Even the smallest changes in the length of the air gap, for example, by temperature effects or changes in the laser wavelength or variations in the mode distribution in the case of multimode fibers (e.g. resulting from fiber bending) give rise to a change in the characteristics of the resonator and thus a variation in the transmission loss which can amount to several tenths of a dB. Such comparatively high loss fluctuations can be intolerable with measuring devices utilizing optical signals and, for this reason, in conventional systems every effort is made to avoid an air gap in the plug connector.
b) Modern semiconductor laser diodes are highly sensitive to reflections. An air gap in the plug connector which can result in about 4% reflection at each glass/air interface cannot be tolerated for most applications of the system. On this ground as well, prior measuring devices required plug-type connectors in which the two fibers were pressed together at least in their respective core regions to ensure a reliable contact between them.
The contacting fiber ends, however, have the disadvantage that even the smallest size dust or dirt particle in the contact region will suffice to cause the formation of a detrimental air gap and to so increase the transmission loss as to interfere with proper operation of the measuring device. In extreme cases, the fiber ends can be damaged.
For conventional plug connectors, therefore, the coupling of the fibers together must be carried out in a dirt-free manner so that before the connection of one light waveguide with another, the ends of the waveguides to contact one another must be carefully cleaned. This is a time-consuming process. With an optical measuring device, plug connections are made and disconnected repeatedly and an optical fiber may have to be connected to a number of devices. The cleaning operations involved and the danger of damage to the fiber ends is thereby greatly increased.
To avoid damage to the fiber ends in regions in which the fibers are to be coupled together in a plug, it is possible to provide between the fiber ends, a fluid light-conducting medium, for example, a so-called index-matching oil. However, the use of the oil has long-term effects which introduce undefinable inaccuracies and limit the reliability of the system.
In the technical manual Feinwerktechnik & Messtechnik 96 (1988) 4, P. 151 to 154, a plug connection has been described which provides a lens arrangement between the ends to be connected by the plug connector. A lens arrangement of this type reduces the effect of contamination in the region of an end face of a plug upon the damping by comparison with direct connection plugs. However this assembly is complex to manufacture and thus correspondingly expensive since the ball-type lenses used must be fabricated with high precision.
Some background as to the problems of earlier connectors can be derived from German patent documents DE 35 06 844 and DE 31 48 562, as well as a publication in the names of Naumann/Schroder "Bauelemente der Optik", Carl Hanser Verlag Munchen Wien 1983, pp. 18, 19, 218, 219.
These publications disclose various optical connecting systems and theoretical considerations of the transmission in the region of the plug connectors.